544 MANUAL OF PHYSIOLOGY. 



of an increasing series of weights placed upon the same area of 

 skin, we can distinguish tolerably accurately between the different 

 weights. It has been found that if a weight of about 30 grammes 

 be placed on the skin a difference of about 1 gramme can be recog- 

 nized that is, we can distinguish between 29 and 30 grammes, 

 if they are applied soon after one another. If the weights em- 

 ployed are smaller, a less difference can be detected ; if larger 

 weights are used the difference must be greater, and it appears 

 that the weight difference always bears the same proportion to 

 the absolute weight used. We can perceive a difference between 

 7* and 7J, 14 J- and 15, 29 and 30, 58 and 60, etc., the discrimi- 

 nating power decreasing in proportion as the absolute degree of 

 stimulation increases. 



One of the reasons why the sense of locality is regarded as dis- 

 tinct from that of pressure is that the latter is found not to be 

 most keenly developed in the same parts where the impressions 

 of locality are most acute. Thus judgment of pressure can be 

 more accurately made with the skin of the forearm than the 

 finger tip, which is nine times more sensitive than the former to 

 ordinary tactile impressions, and the skin of the abdomen has 

 an accurate sense of pressure though deficient in ordinary tactile 

 sensation. 



It has been said above that the weights by which pressure 

 sense is to be tested should be applied rapidly one after the other. 

 This fact depends upon the share taken in the mental judgment 

 by the function we call memory. In a short time the recollection 

 of the impression passes away and there no longer exists any 

 sensation with which the new stimulation can be compared. 



At best we can form but imperfect judgments of pressure by 

 the skin impressions alone. When we want to judge the weight 

 of a body we poise it in the free hand, which is moved up and 

 down so as to bring the muscles which elevate it into repeated 

 action. Hereby we call into action a totally different evidence, 

 namely, the amount of muscle power required to raise the weight 

 in question, and we find we can arrive at much more accurate 

 conclusions by this means. The peculiar recognition of how 

 much muscular effort is expended is commonly spoken of as 



